This invention involves improvements to internal combustion engines and in particular OPOC engines of the type described and claimed in earlier U.S. Pat. Nos. 6,170,443, and 7,434,550, which are incorporated herein by reference. Other types of OPOC engines having one or more crankshafts, also can benefit from the present invention.
As background, the OPOC engine from U.S. Pat. No. 6,170,443 is shown in FIGS. 1 and 2. In those figures, the engine configuration is shown to comprise a left cylinder 100 (1100 FIG. 2), a right cylinder 200 (1200), and a single central crankshaft 300 (1300) located between the cylinders. The left cylinder 100 has an outer piston 110 and an inner piston 120, with combustion faces 111 and 121 respectively, the two pistons forming a combustion chamber 150 between them. The right cylinder 200 similarly has an outer piston 210, an inner piston 220, with combustion faces 211 and 221 and combustion chamber 250. Each of the four pistons 110, 120, 210, and 220 are connected to a separate eccentric on the crankshaft 300 (1300).
The inner piston 120 of the left cylinder 100 is connected to crankshaft eccentric 312 by means of pushrod 412; the inner piston 220 of the right cylinder 200 is similarly connected to crankshaft eccentric 322 by pushrod 422. During normal engine operation, pushrods 412 and 422 are always under compression. The pushrods have concave ends 413 and 423 which ride on convex cylindrical surfaces 125 and 225 on the rear of the inner pistons.
The outer piston 110 of the left cylinder 100 (1100) is connected to crankshaft eccentric 311 by means of pullrod 411 (1411); the outer piston 210 of the right cylinder 200 (1200) is similarly connected to crankshaft eccentric 321 by pullrod 421 (1421). During normal engine operation, pullrods 411 (1411) and 421 (1421) are always under tension. While single pullrods are shown on the near side in FIGS. 1 and 2, it should be understood that pairs of pullrods are used, with one pullrod on the near side of each cylinder and one on the far side of each cylinder. The near and far side pullrods connect to separate crankshaft journals having the same angular and offset geometries. The pullrods 411 (1411) and 421 (1421) communicate with the outer pistons by means of pins 114 (1114) and 214 (1214) that pass through slots (1115) and (1215) in the cylinder walls
The four pistons 110, 120, 210, and 220 have a plurality of piston rings 112, 122, 212, and 222, respectively, located behind the combustion faces. Additional piston rings may be added to the piston skirts, as may be required to reduce wear and control lubrication oil distribution. The cylinders 100 and 200 each have intake, exhaust, and fuel injection ports. On the left cylinder 100, the outer piston 110 opens and closes intake ports 161 (intake piston) and the inner piston 120 opens and closes exhaust ports 163 (exhaust piston). Fuel injection port 162 is located near the center of the cylinder. On the right cylinder 200, the inner piston 220 opens and closes intake ports 261 and the outer piston opens and closes exhaust ports 263. Again, fuel injection port 262 is located near the center of the cylinder. The asymmetric arrangement of the exhaust and intake ports on the two cylinders serves to help dynamically balance the engine, as described below.
Each of the four crankshaft eccentrics 311, 312, 321, and 322 are positioned with respect to the crankshaft rotational axis 310. The eccentrics for the inner pistons 312, 322 are further from the crankshaft rotational axis than the eccentrics for the outer pistons 311, 321, resulting in greater travel for the inner pistons than for the outer pistons. The eccentrics for the inner left piston 312 and the outer right piston 321, the pistons which open and close the exhaust ports in the two cylinders, are angularly advanced, while the eccentrics for the outer left piston 311 and inner right piston 322 are angularly retarded (note that the direction of crankshaft rotation is counterclockwise, as indicated by the arrow in FIG. 1).
As further shown in FIGS. 1 and 2, each cylinder is supercharged. Supercharging improves scavenging, improves engine performance at low rpms and recovers energy from the engine exhaust.
As mentioned above, the pullrods are always under tension forces Fr that are communicated to and from the piston (via piston pins) as compression forces Fp. During the times that the pullrods are at an angle with respect to the reciprocating axis of the outer pistons, there are minor side force components Fs generated at the outer piston pins 114 (1114) and 214 (1214). These side forces occur during both the power and compression strokes of the engine cycle and are directed towards the cylinder walls. Several efforts have been made to minimize the effects of such side forces, including increasing the lubrication between the cylinder wall and the piston skirt; providing more piston rings along the piston skirt; and reducing the length of the piston skirt. However, each conventional attempt to reduce the effects of pullrod side forces has resulted in other undesirable effects.